The present invention relates to methods and apparatus for reducing drag in fluid flow through pipes, and more particularly to a gas ejector and a method of using a gas ejector to provide microbubbles to reduce fluid drag in pipe flow.
Fluid flow in a pipe or a channel occurs ubiquitously in domestic or industrial settings. Flow can be driven by pumps providing a pressure head that overcomes the wall friction or the drag in the flow. For a given flow rate, an increase in pressure head requires an increase in pumping energy, causing a corresponding increase in the cost of operation. Thus, for a given flow rate, a decrease in drag force, resulting in a decrease in pressure head, is a desired operating strategy. Drag reduction for liquid flow in a pipe or channel flow is commonly achieved by adding chemicals such as surfactants or polymers to the liquid. Through the formation of surfactant micelles or polymer chains in the bulk liquid, the frequency of formation and size of the turbulence eddies can be dampened. This results in the boundary layer in the pipe wall becoming less turbulent, resulting in less drag in the liquid flow.
While the use of chemicals is effective in reducing drag, the chemicals can be costly and environmentally unfriendly. One approach for drag reduction is to render the wall boundary layer to be more “slippery,” allowing fluid to be more effectively transported across the wall surface coupled with damping of turbulence eddies yielding a flow to be more laminar. The concept of using microbubbles to reduce the skin friction of a surface has been reported in the literature for ship hull applications. The flow velocities in these applications are typically high at from 4 to 20 m/sec and are normally in the Reynolds number range of 0.3×106 to 1.6×106. Reynolds number is a dimensionless number that is an indicator of the type of fluid flow, either laminar, transitional, or turbulent. Generally, Reynolds numbers of less than about 2500 are indicative of laminar fluid flow. Ship hull applications using microbubbles for drag reduction occur at rather high relative solid surface velocities over an infinite stationary liquid. Also, liquid turbulence decreases quickly at relatively short distances from the hull surface.
In a significantly different fluid flow condition, pipe flow occurs at rather low relative liquid flow velocities over a stationary solid surface with liquids well confined by the solid surface and with turbulence occurring throughout the liquid medium during flows. Very little, however, is known about the role of microbubbles and their drag reducing effects in a pipe flow. Specifically, the mechanism of the microbubble drag reduction phenomenon and microbubble-wall interactions are not understood well enough to allow proper design and operation of microbubble systems for a pipe flow with flow velocities of less than about 4 m/sec and in Reynolds numbers of less than about 1.0×106.
Accordingly, there remains a need for methods and apparatus for reducing drag in fluid flow through pipes that are cost effective and environmentally friendly.